Stoichiometry: Quantitative Information About Chemical Reactions

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Stoichiometry Quantitative Information About
Chemical Reactions

• Chapter 4

2
Learning Objectives

• Students understand
• The law of the conservation of matter, which
  forms the basis of stoichiometry.
• The meaning of a limiting reactant in a chemical
  reaction.

3
Learning Objectives

• Students will be able to
• Perform stoichiometric calculations
• Calculate theoretical and percent yields
• Analyze mixtures of compounds or determine the
  formula
• Define and use concentration
• Use the relationship of pH and H3O
• Use spectrophotometry to determine concentration

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4.1 Mass Relationships

• stoichiometric factor a mole ratio based on the
  coefficients of reactants or products in the
  balanced equation
• What mass of oxygen, O2, is required to
  completely combust 454 g of propane, C3H8? What
  masses of CO2 and H2O are produced?
• C3H8 5O2 ? 3CO2 4H2O

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Practice Problem

• Aluminum reacts with Cl2 to form AlCl3. If you
  have 0.40 mol of Al, what amount (moles) of Cl2
  is needed for a complete reaction?
• Write the equation for the reaction between
  magnesium and oxygen. If you want 1.0g of MgO,
  what mass of Mg should you use?

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4.2 Limiting Reactants

• A limiting reactant determines the amount of
  product formed.
• Step 1 Find the amount of each reactant.
• Step 2 What is the limiting reactant?
• Step 3 Calculate the mass of product.
• Step 4 Calculate the mass of excess reactant.

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Limited Reactants

• Pure silicon, required for computer chips and
  solar cells, is made by the reaction
• SiCl4 2Mg ? Si 2MgCl2
• If you begin with 225 g each of SiCl4 and Mg,
  which is the limiting reactant in this reaction?
  What quantity of Si, in grams, can be produced?

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4.3 Percent Yield

• yield actual yield x 100
• theoretical yield
• Methanol, CH3OH, can be burned in oxygen to
  provide energy, or it can be decomposed to form
  hydrogen gas, which can then be used as a fuel.
• CH3OH ? 2H2 CO
• If 125 g of methanol is decomposed, what is the
  theoretical yield of hydrogen? If only 13.6 g of
  hydrogen is obtained, what is the percent yield
  of the gas?

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4.4 Quantitative Analysis

• A substance of unknown concentration may be
  allowed to react with a known quantity of
  something else
• OR
• An unknown substance can be converted to one or
  more substances of known composition (back track)

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Chemical Analysis

• One method for determining the purity of a sample
  of titanium (IV) oxide, TiO2, an important
  industrial chemical, is to combine the sample
  with bromine trifluoride.
• 3TiO2 4BrF3 ? 3TiF4 2Br2 3O2
• Oxygen gas is evolved quantitatively. The gas
  can be captured readily, and its mass can be
  determined. Suppose 2.367 g of a TiO2-containing
  sample evolves 0.143 g of O2. What is the mass
  percent of TiO2 in the sample?

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Determining an Unknown Formula

• The unknown pure compound is decomposed into
  known products.
• The reaction products are isolated in pure form
  and the amount of each is determined.
• The amount of each product is related to the
  amount of each element in the original compound
  to give the empirical formula.

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Determining an Empirical Formula

• A 0.523 g sample of the unknown compound CxHy is
  burned in air to give 1.612 g of CO2 and 0.7425 g
  of H2O. A separate experiment gave a molar mass
  for CxHy of 114 g/mol. Determine the empirical
  and molecular formulas for the hydrocarbon.

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Practice Problem

• A 0.509 g sample of an unknown organic compound
  containing C, H, and O was burned in air to give
  1.316 g of CO2 and 0.269 g of H2O. What is the
  empirical formula of this compound?

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4.5 Concentrations of Compounds in Solution

• 1 mole element 6.02 x 1023 atoms
• 1 mole molecular compound 6.02 x 1023 molecules
• 1 mole ionic compound 6.02 x 1023 formula units

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Molarity

• Molarity moles of solute per liter of solution
• M mol/L
• Example How would you prepare 2.00L of 0.250M
  NaOH from solid NaOH?

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Molarity

• Sodium bicarbonate, NaHCO3, is used in baking
  powder formulations and in the manufacture of
  plastics and ceramics, among other things. If
  26.3 g of the compound is dissolved in enough
  water to make exactly 200mL of solution, what is
  the molar concentration of NaHCO3? What are the
  concentrations of the ions?

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Preparations of Known Solutions

• An experiment in your laboratory requires 250.mL
  of a 0.0200M solution of AgNO3. You are given
  solid AgNO3, distilled water, and a 250.mL
  volumetric flask. Describe how to make up the
  required solution.

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Dilutions

• M1V1 M2V2
• How would you prepare 2.00L of 0.250M NaOH from
  1.00M NaOH?

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Preparing a Solution by Dilution

• In one of your laboratory experiments, you are
  given a solution of CuSO4 that has a
  concentration of 0.15M. If you mix 6.0mL of this
  solution with enough water to have a total volume
  of 10.0mL, what is the concentration of CuSO4 in
  this new solution?

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Preparing a Solution by Dilution

• An experiment calls for you to use 250. mL of
  1.00M NaOH, but you are given a large bottle of
  2.00M NaOH. Describe how to make the 1.00M NaOH
  in the desired volume.

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4.6 pH, a Concentration Scale for Acids and Bases

• pH can be calculated directly from hydrogen ion
  concentration
• pH -logH3O
• Can use the antilog of pH to figure out hydrogen
  ion concentration
• H3O 10-pH

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pH

• What is the pH of a solution of HCl, where HCl
  2.6 x 10-2 M?
• What is the hydrogen ion concentration in
  saturated calcium hydroxide with a pH of 12.45?
• A 0.365 g sample of HCl is dissolved in enough
  water to give 2.00x102 mL of solution. What is
  the pH?

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4.7 Stoichiometry of Reactions in Solution

• If you combine 75.0 mL of 0.350M HCl and an
  excess of Na2CO3, what mass of CO2 (in grams)
  should be produced?

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Titrations

• A titration can be used to determine the
  concentration of an unknown solution.
• A substance, present in unknown quantity, can be
  allowed to react with a known quantity of another
  substance.
• If the stoichiometric ratio for their reaction is
  known, the unknown quantity can be determined.

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Acid-Base Titrations

• Acid-base indicators are used to tell when the
  reaction is complete.
• When the solution changes color, the equivalence
  point (also called end point) has been reached.
  The amount of OH- equals the amount of H.
• Amount of base (mol) conc. of base (M) x volume
  of base (L)

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Acid-Base Titration

• A 25.0 mL sample of vinegar requires 28.33 mL of
  a 0.953 M solution of NaOH for titration to the
  equivalence point. What mass (in grams) of
  acetic acid is in the vinegar sample, and what is
  the concentration of acetic acid in the vinegar?
• CH3CO2H NaOH ? NaCH3CO2 H2O

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Standardizing an Acid or Base

• Often the concentration of the base is unknown
  and must be determined through standardization.
• primary standard a sample of solid acid or base
  which is titrated with a solution of the base or
  acid to be standardized
• titrate with another solution that is already
  standardized

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Standardization

• Hydrochloric acid can be purchased from chemical
  supply houses with a concentration of 0.100M, and
  such a solution can be used to standardize the
  solution of a base. If titrating 25.00 mL of a
  sodium hydroxide solution to the equivalence
  point requires 29.67mL of 0.100M mL, what is the
  concentration of the base?

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Practice Problem

• An unknown monoprotic acid reacts with NaOH
  according to the net ionic equation
• HA(aq) OH-(aq) ? A-(aq) H2O(l)
  Calculate the molar mass of HA if 0.856 g of the
  acid requires 30.08 mL of 0.323 M NaOH.

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Redox Titrations

• A sample containing vitamin C is titrated with
  Br2, an oxidizing agent.
• C6H8O6 Br2 ? 2HBr C6H6O6
• A 1.00g chewable vitamin C tablet requires
  27.85mL of 0.102M Br2 for titration to the
  equivalence point. What is the mass of vitamin C
  in the tablet?

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Practice Problem

• Case Study p. 173 Forensic Chemistry
  Titrations and Food Tampering

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4.8 Spectrophotometry

• It is possible to measure the extent of light
  absorption through a solution that is colored,
  and then you can relate this to the concentration
  of the dissolved solute. This method of
  quantitatively measuring light absorption is
  called spectrophotometry.

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Spectrophotometric Analysis

• Record the absorption spectrum of the substance
  to be analyzed.
• Choose the wavelength for the measurement.
• Prepare a calibration curve (or plot).
• Determine the concentration of the species of
  interest in other solutions.

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Transmittance and Absorbance

• Transmittance is the ratio of the amount of light
  transmitted by or passing through the sample
  relative to the amount of light that initially
  fell on the sample (the incident light).
• Absorbance is defined as the negative log of the
  transmittance. As absorbance increases,
  transmittance decreases. Absorbance increases as
  concentration increases.

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The Beer-Lambert law

• Absorbance increases as path length increases.
• The amount of light absorbed by a solution can be
  used to determine the concentration of the
  absorbing molecule in that solution.
• A e x l x c
• where e is the molar absorptivity

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Practice Problem

• Using the data on p. 177 Check Your
  Understanding, create a calibration curve for
  Cu2(aq). Write the equation for your line and
  calculate the concentration of Cu2(aq) in a
  solution if the absorbance is 0.528.

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Homework for Chapter 4

• After reading chapter 4, you should be able to do
  the following
• P. 179g (80-81, 87, 89, 98-99, 117, 125, 132-133)